Numerous studies indicate that CD4 T cells are required for acute cardiac allograft rejection. However, the precise role for CD4 T cells in this response has remained ambiguous due to the multi-potential properties of this T cell subpopulation. We have recently demonstrated the capacity of CD4 T cells to serve as direct effector cells of cardiac allograft rejection. CD4 T cells are both necessary and sufficient for acute graft rejection as indicated by adoptive transfer experiments in immune-deficient SCID and rag-/- recipients. Acute CD4 T cell-mediated rejection requires MHC class II expression by the allograft, implicating the importance of direct graft recognition in this response. While CD4 T cells can serve as effector cells for primary acute cardiac allograft rejection via direct donor recognition, they can also serve as helper cells via the indirect response leading to the function of other graft-reactive lymphoid populations. The difficulty in examining the relative contribution of these CD4-dependent pathways to acute allograft rejection is that these responses normally occur simultaneously. In the adoptive transfer model systems proposed, the consequence of direct (donor MHC-restricted) and indirect (host MHC-restricted) CD4 reactivity can be studied independently. Thus, the overall goals of this proposal are to: (1) Clarify the nature of acute CD4-mediated cardiac allograft rejection via direct donor recognition, and (2) Clarify the pathway of 'indirect' CD4 reactivity that results in help for other graft-reactive CD8 T cells and/or B cells. These general goals will be examined through the following specific aims: I) Define the recognition and costimulatory requirement(s) of direct CD4-dependent cardiac allograft rejection. This aim will test the novel hypothesis that antigen recognition and costimulation can be delivered by different APCs in vivo. II) Determine the effector mechanism(s) of CD4-dependent direct cardiac allograft rejection. This aim will test the hypothesis that direct, CD4-mediated rejection is cytokine-dependent in vivo. III) Determine the nature of indirect CD4 T cell 'help' for cardiac allograft rejection. This aim will test the hypothesis that indirect CD4 T cells can provide 'unlinked' T cell help for graft reactive CD8 T cells. The information obtained will be useful in understanding the basic immune mechanisms of human cardiac transplantation.